In psychometricpsychology, fluid and crystallized intelligence (abbreviated gF and gC, respectively) are factors of general intelligence identified by Raymond Cattell (1971). Fluid intelligence is the ability to find meaning in confusion and solve new problems. It is the ability to draw inferences and understand the relationships of various concepts independent of acquired knowledge (Cavanaugh & Blanchard-Fields, 2006). Crystallized intelligence is the ability to utilize previously acquired knowledge and experience (Cavanaugh & Blanchard-Fields, 2006). Tacit knowledge falls into the crystallized intelligence category. Tests of intelligence are intended to examine both types of intelligence. For example, the WAIS measures fluid intelligence on the performance scale and crystallized intelligence on the verbal scale (Lee, et al., 2005). The terms are somewhat misleading because one is not a "crystallized" form of the other. Rather, they are separate though correlated mental abilities.

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Fluid and crystallized intelligence are described as discrete factors of general intelligence, or g (Cattell, 1987). Charles Spearman (1927), who originally developed the theory of of the g, made a similar distinction between eductive and reproductive mental ability. It should be noted that Spearman's original work was harshly criticized and refuted by Binet (1905). In his critique, Binet goes as far as to say that Spearman actually fabricated his data, or at the least manipulated the data to support his hypothesis. In any case, Cattell (1987) continued Spearman's work and developed the concepts of fluid and crystallized intelligence. According to Cattell (1987), "...it is apparent that one of these powers… has the 'fluid' quality of being directable to almost any problem. By contrast, the other is invested in particular areas of crystallized skills which can be upset individually without affecting the others." Thus, his claim was that each type, or factor, was independent of the other, though many authors have noted an apparrent interdependence of the two (Cavanaugh & Blanchard-Fields, 2006).

Fluid intelligence includes such abilities as problem-solving, learning, and pattern recognition. As evidence for its continuity, Cattell suggests that gF abilities are rarely affected by brain injuries. The Cattell Culture Fair IQ test, the Raven Progressive Matrices, and the performance subscale of the WAIS are measures of gF.

Crystallized intelligence is possibly more amenable to change as it relies on specific, acquired knowledge. For example, a child who has just learned how to recite the fifty states of America now owns a new piece of crystallized intelligence; but his or her general ability to learn and understand gF has not been altered. An example of the flexibility, or ability to revise, crystallized intelligence can be seen in beliefs about Santa Claus. A five year-old child may believe that Santa Claus lives at the North Pole. Later, when the child is eight years old, he learns there is no Santa Claus. His belief that Santa lives at the North Pole was then invalidated and new knowledge is gained, there is no Santa Claus. The prior knowledge was revised in order to accommodate the new learning. Vocabulary tests and the verbal subscale of the WAIS are considered good measures of gC.

Not surprisingly, people with a high capacity of gF tend to acquire more gC knowledge and at faster rates. This is sometimes called investment. Researchers have found that criminals have disproportionately low levels of crystallized intelligence, possibly as a result of them investing their ability into skills that are not measured on IQ tests.

Fluid intelligence generally correlates with measures of abstract reasoning and puzzle solving. Crystallized intelligence correlates with abilities that depend on knowledge and experience, such as vocabulary, general information, and analogies. Paul Kline (1998) identified a number of factors that shared a correlation of at least r=.60 with gF and gC. Factors with median loadings of greater than 0.6 on gF included induction, visualization, quantitative reasoning, and ideational fluency. Factors with median loadings of greater than 0.6 on gC included verbal ability, language development, reading comprehension, sequential reasoning, and general information. It may be suggested that tests of intelligence may not be able to truly reflect levels of fluid intelligence. Some authors have suggested that unless an individual was truly interested in the problem presented, the cognitive work required may not be performed because of a lack of interest (Messick 1989, 1995). These authors contend that scores on tests that measure fluid intelligence may reflect more of a lack of interest in the tasks rather than the ability to complete the task successfully.

Fluid intelligence, like reaction time, peaks in young adulthood and then steadily declines. This decline is possibly due to local atrophy of the brain in the right cerebellum (Lee, et al., 2005). Cavanaugh and Blanchard-Fields (2006) also indicate that a lack of practice, along with the age related changed in the brain may contirbute to the decline. Crystallized intelligence increases gradually, stays relatively stable across most of adulthood, and then begins to decline after age 60. This decline, however, is not generally detectable before age 65 (Cavanuagh & Blanchard-Fields, 2006).

According to recent research, gF and gC can be traced to two separate brain systems. Fluid intelligence involves the dorsolateral prefrontal cortex, the anterior cingulate cortex, and other systems related to attention and short-term memory. Crystallized intelligence appears to be a function of brain regions that involve the storage and usage of long-term memories, such as the hippocampus (Geary, 2005). It should be noted that even though it has been suggested that fluid intelligence has a locus in the frontal lobe, Lee, et al. (2005) found no correlation between declines in fluid intelligence and frontal lobe changes.